In recent years, application of infinite solar light producing no harmful substances has been actively studied. As those in which a solar light is presently put into practical use as a clean energy source, there are inorganic type solar cells such as single crystalline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride and indium copper selenide for domestic use.
However, there are drawbacks in these inorganic type solar cells. Extremely high purity is required, for example in the case of the silicon type, and the purification process is complicated and includes many steps, resulting in high production cost.
On the other hand, many solar cells employing an organic material have also been proposed. Examples of the organic solar cell include a Schottky type photoelectric conversion element in which a p-type organic semiconductor and metal having a small work function are joined, and a hetero-junction type photoelectric conversion element in which a p-type organic semiconductor and an n-type inorganic semiconductor or a p-type organic semiconductor and an electron accepting organic compound are joined. The utilized organic semiconductors are synthesized dyes or pigments such as chlorophyll, perylene and so forth, conductive polymer materials such as polyacetylene and so forth, and composite materials thereof. These materials are thin-layered by a vacuum evaporation method, a casting method, a dipping method or the like to prepare a cell material. The organic materials have advantages that they are inexpensive and easy to obtain a large dimension, but there is problem that most of them exhibit a low conversion efficiency of 1% or less and poor durability.
In such a situation, a solar cell exhibiting favorable properties has been reported by Dr. Gratzel et al. in Switzerland (see B. O'Regan & M. Gratze, Nature, 353, 737 (1991), for example). The proposed cell is a dye sensitizing type solar cell, and a wet type solar cell in which a titanium oxide porous film spectrally sensitized by a ruthenium complex is provided as a functional electrode. Advantages of this system are that it is not necessary to purify an inexpensive oxide semiconductor such as titanium oxide up to high purity, and wide wavelength regions of visible light can be used, whereby a solar light containing visible light components can be effectively converted into electricity.
However, this system employs the ruthenium complex which is resourceless, and when this system is practically applied to a solar cell, supply of the ruthenium complex is a concern. Further, the ruthenium complex has problem in that it is expensive, and low in aging stability, but this problem can be solved if the ruthenium complex can be replaced with an inexpensive and stable organic dye.
There is disclosure in Japanese Patent O.P.I. Publication No. 2005-63833 that when compounds having a rhodamine nucleus-containing amine structure are employed as a dye for this solar cell, a dye-sensitizing type photoelectric conversion element (organic dye-sensitizing type solar cell) with high photoelectric conversion efficiency is obtained. However, it is supposed that these compounds having in the molecules plural sulfur atoms which contribute to light instability are lower in durability to light. Accordingly, further improvement of a sensitizing dye is required.